<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <div id="app">
        <div :class="classes"></div>
    </div>
    <script src="https://unpkg.com/vue@2.6.1/dist/vue.min.js"></script>
    <script>
        var app = new Vue({
            el: '#app',
            data: {
                size: 'large',
                disable: true
            },
            computed: {
                classes: function () {
                    return [
                        'btn',
                        {
                            ['btn-' + this.size]: this.size !== '',
                            ['btn-disabled']: this.disable
                        }
                    ]

                }
            }
        });
    </script>
</body>
</html>